The California university researcher killed in Ethiopia protests was a brilliant scientist with an infectious smile who was studying the effects of climate change at the time of her death, the colleague who was traveling with her said on Thursday.

Sharon Gray, a 31-year-old postdoctoral scientist at the University of California, Davis, was riding through Addis Ababa with her co-researcher Siobahn Brady when their car was attacked by demonstrators hurling rocks.

Brady was not hurt in the incident and was returning to the United States, according to the university. The U.S. State Department was assisting in bringing Gray's body home.

"She had an infectious smile and giggle, a calm and patient nature and she was truly committed to helping people through her studies of plant biology. I and her colleagues and friends in my lab will miss her intensely," Brady said in a written statement.

"These last hours of her life and the past day have been incredibly difficult and we ask for your respect for the privacy of my lab group, our project members and her colleagues while we mourn," she said.

The two women were the lead researchers on study to understand the response of plants to climate change with the Netherlands Institute of Ecology and were in Addis Ababa for a "kick off meeting," Brady said, calling Gray a future leader in her field.

On her Twitter page Sharon Gray described herself as an "outdoor adventurer, traveler, foodie" as well as a postdoctoral fellow. A memorial page posted by the university was filled with photos of her in fields of crops, rock-climbing and hiking, surrounded by friends.

Originally from the Chicago area, Gray earned her undergraduate degree from the University of Illinois-Champaign.

State Department spokesman John Kirby confirmed that an American citizen had died in Ethiopia on Tuesday but declined to provide further details.

"We offer our sincerest condolences to the family and to the loved ones. We are providing all possible consular assistance. Out of respect for the family, we must decline further comment," he said.

Asked if the United States believed that protesters had targeted Americans, Kirby replied: "As for the situation itself, that's really for the local authorities to speak to in terms of the investigation and how they are looking into it."

The U.S. Embassy in Ethiopia has said that an American woman was killed on Tuesday when stones were hurled at her vehicle on the outskirts of Addis Ababa. Residents there have described crowds attacking vehicles since a stampede at a weekend protest killed at least 55 people.

That stampede began when police fired teargas and shots in the air to disperse anti-government demonstrations during a festival in the Oromiya region, south of the capital, which has been a focus for demonstrations by locals who say land has been seized to build factories and housing blocks.

Protests have also increasingly turned to broader issues of political freedom. Amnesty International has demanded an investigation into how security forces handled the weekend protest.

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